Currently, FDA-licensed pharmaceuticals used to treat rheumatoid arthritis (RA) focus largely on alleviation of symptoms, either through pain management, general immunosuppression, or by antagonizing cytokines such as TNF-?. Despite recent advances in biologic therapies, these treatments do not address the underlying autoimmune condition. Ligand epitope antigen presentation system (L.E.A.P.S.) conjugates are a peptide vaccine platform designed to modulate the immune response in an antigen-specific manner. LEAPS are composed of two peptide components, an immune cell binding ligand (ICBL) and a peptide (epitope) implicated in an infectious or autoimmune disease. The ICBLs include peptide J from human -2 microglobulin, with Th1-polarizing activity, and peptide derG (or G) from human MHC class II chain with Th2 polarizing activity. In mice with collagen-induced arthritis (CIA), a Th17-mediated disease, a J-collagen peptide conjugate reduced disease severity by suppressing pro-inflammatory cytokines and increasing protective cytokines. To date, no derG conjugates have shown protective activity. In the present proposal, we hypothesize that L.E.A.P.S. vaccine modulates the immune response towards a protective condition in a second mouse model of RA, cartilage proteoglycan (PG) induced arthritis (PGIA). An advantage of the PGIA model is that it is either a Th1 or Th17 cytokine dependent arthritis based on the route of induction, intraperitoneal (i.p.) or subcutaneous (s.c.), respectively. Using the dominant arthritogenic epitope PG70 (ATEGRVRVNSAYQDK) of the G1 domain of PG, conjugated to J or derG, preliminary studies in the i.p. PGIA model demonstrate that the derG-PG70 conjugate inhibits ongoing arthritis by shifting the balance from a Th1 to a Th2/Treg response. Based on studies in CIA, we expect that the J-PG70 conjugate will modulate pathogenic Th17 responses in s.c.-induced PGIA toward protective Th1 responses. Efficacy will be assessed by measuring arthritis index, histopathological examination of peripheral joints, antibody production, T cell proliferation, and cytokine responses. In Aim 1, we will compare the LEAPS-PG70 peptide conjugates and subunit peptides for therapeutic efficacy in both the i.p. and s.c. PGIA models. In Aim 2, we will examine the mechanism of action focusing on binding to (and effects on) T cells and dendritic cells for the two conjugates using cells from naive PG-TCR-transgenic and PG-immunized (i.p. or s.c.) wild type mice. After success in phase I SBIR, we will advance the L.E.A.P.S vaccine to a phase II SBIR, with the initial goal of further characterizing the mechanisms by which LEAPS peptides act on immune cells from mice and humans, and with the ultimate goal of optimizing the vaccine for the treatment of RA.